We are looking at a 96 GMC 6.5l turbo diesel that has a knock and smokes
bad... someone said possibly turbo any other ideas ??? They said the bad
turbo allows oil to be sucked into the motor causing the detonation (knock
sound) and the oil smoke... is this possible ? They have a video of it
starting and running at
To me it's more of a knock than detonation... I thought it sounded more
solid of a knock. (rod or piston slap) I know rod usually only makes noise
on the unload period, this is more during a load, (while feeding into). Any
help would be appreciated...
Your simptons match a bad injector pump or one that it out of time.
Timing is extremely critical as it can knock and run badly and smoke
it set wrong or it is worn out. The injection pump is the heart of a
6.5, without its proper function you will have one that runs poorly or
not at all.
Based on the year and average mileage for this motor you should check the
Fuel injection pump - stretched timing chain (not uncommon) or improperly
timed/calibrated if it has been recently worked on or replaced. The pump
could also be bad with a bad fuel solenoid driver (FSD) and/or a fried
optical sensor, but this would more than likely give you a no-start or
stalling condition, espeically when hot.
Fuel injectors - worn or damaged, or replaced with higher flow marine grade
injectors without proper additional modifications (dumping too much fuel
into the cylinders).
If this truck is a 1500 series (half ton) it will have an EGR valve which
may also be on the fritz, have an internal leak or bad gaskets. A bad valve
or control solenoid could be cooling the combustion event by too much
causing a certain amount of fuel to go unburned (a cause of black smoke).
A bad turbo could also cause similar issues, especially if the bearings are
shot and it is leaking oil into the intake manifold. The turbo dump valve
could be jammed shut which may cause smoking, but I don't think it would
cause a knock. Open up the plumbing between the turbo and the intake
manifold to see if it's full of oil from the turbo and check to see that you
can move the dump valve by hand with the vacuum tubing disconnected from the
The other issue it may have could also be burn-through of the head gasket
around the rear-most cylinders, an issue for pre '98 motors used for lots
of hauling and towing from excessive heat. This may cause smoke but likely
not black smoke. Check to see what the oil looks like.
Any way you cut this isn't going to be a cheap problem to fix. Heck, a fuel
injection pump alone could run you around $1800.00. If you believe there
are multiple issues (especially if you don't have a good idea what they are
yet) then my personal opinion is that unless you really, really, really love
this truck and are willing to put in what potentially could be a lot of time
and money then I would pass it by and look for something in better
condition. I am a fan of Chevy/GMC diesels, but unless you know exactly
what you are getting into then you could end up sinking more money into this
truck that you paid for it to begin with.
Good luck - Jonathan
In this case a streched timing chain would retard pump timing and may
cause it to smoke and lack power but not knock a lot so I do not see
this as a issue here.
I took the time today to download and watch the video the original poster
listed (I tried earlier and it wouldn't). The hard starting and bad idle
just scream injection pump, but not knowing anything about the pump
(original vs. replacement, etc.) I can't say for sure if it's just from
excessive wear on the original or a bad installation and calibration on a
rebuilt unit. The smoke is black on start up but the video quality isn't
good enough to see if it remains fully black as it runs or turns white or
gray. Just as a precaution I would do a leak down and compression test on
the cylinders (especially the rear ones where gasket problems are a known
issue) but I'm still leaning towards the FI pump. The engine knock is
terrible - again the FI pump timing, either from wear on the timing set or a
bad install. I've never seen one of those chains jump a tooth on their own,
but I have seen folks screw up the timing by not having it calibrated after
reinstalling the pump or installing a new one thinking it wasn't needed.
Cheers - Jonathan
I pretty much bailed on buying it... you had a few god points with sinking
too much money into it. I think I'll keep my eye open for one in better
running condition. I am a mechanic and have many years in the field but I
got out of it in the early 90's... for the most part now I mess with my old
cars and our equipment. Just rebuilt our Kubota diesel in our New Holland
skidsteer... I'll stick to simple diesel stuff like that and my race
engines... Thanks for the help you both definitely steered me in the right
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